The invention comes within the field of electromagnetic inspection of metal products by Foucault or eddy currents in order to detect surface defects.
A number of electromagnetic methods of probing surface defects are already known. They are largely based on a saturation magnetization of the product, the latter producing a gap in the magnetic field at the location of the defects as well as the appearance of leak lines. The heterogeneities of the field can then be materialized by sprinkling with iron filings, which accumulate at the defect locations. This method, designated by the term magnetoscopy, is quite sensitive, but does not yield good results on scaled products, furnishes no indication of the depth of the defects and is carried out almost whollymanually, especially in the locating of defects, which is visual. Optical detection is based on the same principle, but the use of a colored magnetic powder makes possible the observation of the product by means of a television camera and the locating of defects can be automated. The leakage flux method consists of measuring the magnetic field by use of Hall probes; it is hardly suitable for anything but scaled products, but, for the latter, it is relatively quantitative and makes it possible to estimate the extent of the defects; it can also be easily automated. The main disadvantages of all of these techniques are that they can be applied, under satisfactory sensitivity conditions, only to prescaled products and require premagnetization and then demagnetization of the product after probing. These two stages consume energy in quantity variable with the product, but always high.
Another technique, of which the procedure according to the invention forms part, is based on the use of foucault or eddy currents. In fact, the impedance of a coil electromagnetically coupled with the product examined varies with a number of parameters linked to the latter, such as its magnetic permeability (if one is below the Curie point in the case of steel), its electric resistivity, the presence of defects and coil-product distance. This technique, like the leakage flux method, offers the advantage of making possible an estimate of the extent of the defects. However, it generally also has the disadvantage of requiring saturation magnetization of the products, in order to eliminate local variations of magnetic permeability, and then their demagnetization.